This invention relates to a developing device for use in an electrophotographic copying machine, and more particularly to a developing device for developing an electrostatic latent image formed on an electrostatic image support member into a multicolored toner image.
In order to develop an electrostatic latent image formed on an electrostatic image support member into a mutlicolored toner image according to the electrophotography, there has been proposed arrangements in which a plurality of magnetic brush developing units are disposed along the electrostatic image support member for successive or sequential development.
This type of arrangement of necessity requires that some of the developing units be held inoperative while another is carrying out image development. A problem associated with this is that developer fed to an inoperative magnetic brush developing unit may unwantedly contact the surface of the electrostatic latent image support member, resulting in the spoiling of the toner image.
To overcome the above shortcoming, two different types of developing devices have been proposed; one is such that among a plurality of developing units, developing rolls of the developing units which are in an inoperative condition are diverted or displaced from the electrostatic image support member, so as to avoid spoiling the toner image; and the other is such that a control plate for controlling a flow rate of developer is provided in each developing unit, so as to interrupt the feeding of a developer to developing rolls of the inoperative developing units, thereby preventing the toner image from being spoiled.
These developing devices, however, have been accompanied with other drawbacks. In the former, extremely close control of several process parameters is required to obtain optimum development. In addition, the control arrangement is complex in construction. In the latter, a small amount of developer remains on developing rolls of inoperative developing units, this residual developer being extremely difficult to promptly remove from the roll. Furthermore, the small amount of residual developer on the developing rolls tends to be more concentrated along the magnetic lines of force, and contact the surface of the electrostatic image support member in those areas, thus forming undesired stripes or lines on the toner image developed by the other magnetic brush developing unit, thereby spoiling the appearance of the toner image.